AC problem or insulation problem?

So I live in an ancient house, no insulation and no room for insulation in the walls, giant gaps between the door frame and door (I'm working on that), etc.

In my area daily highs are around 95-100 degrees Fahrenheit. The A.C. currently runs nonstop all day long and manages to keep the house at an unbearable (for me, a fat guy that sweats a lot) 88 during the day and down to 82ish just before dawn. Electricity bills are at $500 per month, compared to $200 in the winter when we didn't have heat running at all.

I moved in near the middle of last summer and was too busy to pay attention to the A.C much, but I'm pretty sure we had it set to 78 or so and it could get it down to that temperature without running nonstop. Viewing my electricity bills seems to confirm this as we were paying around $350 for the same months last year.

So is there any way to identify if my A.C is not working correctly? Cold air is blowing out of the vents, scientifically measured at around 67 degrees by sticking a meat thermometer into the vent

I'm a fairly competent diyer, I work on my car and with electronic devices as a computer engineering major, so I'm not afraid to open things up and tinker with them, but A.C. units is just uncharted water for me, if someone could point me to some guides and schematics I would be very grateful...

AC problem or insulation problem?

Don't know house sq ft, but it is 5 br kitchen and pretty large hallway.

I'm aware that more insulation would help immensly, but the house was built with no space in the walls for it, I'm only planning on living there for 1 or 2 more years so major rennovations just would not be cost efficient and I'm limited to minor door/window insulation.

We also have several beefy (and hot) computers running pretty much all day which don't help.

The crux of the problem though is that the A.C. seems to be performing worse than last summer, I'm just trying to figure out if there is a problem with it before I call the landlord and ask her to send someone out.

How cold should the air be coming out of the vents? It is on the cold side of cool, but it seems to me like it should be colder - don't know what it was like before though since I never checked it before now. Like I said, my super scientific experiment of sticking a meat thermometer down the vent shows that the temp is around 67.

I can't seem to find any possible problems online that would cause the AC to work but not at full capacity - anyone know what could cause this?

As stated, I don't mind getting inside the unit and am in fact quite interested in learning about the mechanics of an AC unit, I just don't know where to start (besides flipping the breaker of course :D)